Fedora 18 Beta shim is not signed as FESCo agreed not to block Beta on Secure Boot support (btw. it's currently blocked on legal).
For Beta, you should disable Secure Boot in the UEFI settings. For final, the decision will be based on the resolution of https://fedorahosted.org/fesco/ticket/978 ticket.
*** This bug has been marked as a duplicate of bug 872272 ***